The invention relates to a spring return mechanism, particularly a spring return mechanism for an axial piston machine which is operable either as a pump or as a motor. Axial piston machines may be described as having two components one a cylinder block in which pistons operate and the other a plate or disc (oblique disc) against which the pistons bear. Operation of the machine is effected by rotating one of the components relative to the other or by effecting rotation of one or both the components by applying a fluid under pressure to the cylinders. In adjustable machines, one of the components is supported for relative swinging or rocking movement with respect to the other component from a neutral position where the disc or plate is perpendicular to the axis of the cylinders. The movement of the rockable component from neutral produces relative inclination of the plate and the cylinder block axis to adjust the delivery or power of the machine. In operation one or both of the components may be rotating and when the components are in their neutral position the plate and the cylinder block have the same axis which is also the axis of rotation. This may be termed the axis of the machine.
Commonly, the relatively swingable components of the axial piston machine are spring biased to their neutral, i.e. zero stroke position. It is important that the spring biasing act to return the components to the zero stroke position since if this is not done accurately, power may be inadvertently transmitted by the machine when the prime mover for effecting operation of the machine, for example, an internal combustion engine connected to rotate one of the components, is started. In known spring return devices for such machines, the spring engages a lever arm off the axis of swinging of the adjustable component. In many designs, there is a spring pocket on each side of the swinging axis and commonly the spring pockets are located within hydraulic cylinders of a servo mechanism for effecting the swinging. Such an arrangement is extremely costly and requires much construction, space, and weight, and above all, necessitates joints in which a play can occur. Also such arrangements are hard to get at from outside for the purpose of adjustment. In a known spring return mechanism for the swinging of a housing, there is a spring coaxial with the serving setting piston. This spring is stretched between stops so that with each displacement of the servo piston from the zero stroke position thereof and regardless of the direction of movement of the servo piston, the spring is compressed. (See publication of Linde AG, Guldner Aschaffenburg Division) Here also, a joint part is still needed between the spring and swinging part at which play can occur.